The present invention relates to a display, in particular, to an emissive display using organic electroluminescent (EL) devices.
The application of the organic EL devices to a plane type display is promoted, and it is proposed to realize an active matrix display with high brightness. As regards the driving system using a low temperature polysilicon thin film transistor(TFT), it is described in SID 99 Technical Digest, pp. 372-375.
In the pixel structure, a scan line, a signal line, an EL power supply line, and a capacitance reference voltage line are arranged to intersect with one another, and in order to drive the EL device, a holding circuit of a signal voltage is formed by an n-type scan TFT and a storage capacitor. The held signal voltage is applied to a gate of a p-channel type driving TFT, and controls a conductance of a main circuit of the driving TFT. The main circuit of the driving TFT and the organic EL device are connected in series from the EL power supply line and connected to an EL common line.
In driving this pixel, a pixel selection pulse is applied from the scan line, and the signal voltage is written to the storage capacitor through the scan TFT, and is held. The held signal voltage is applied as the gate voltage of the driving TFT, and controls a drain current, according to a conductance of the driving TFT determined by a source voltage-supplied from the power supply line, and a drain voltage, and a driving current of the EL device is controlled, thereby controlling the display brightness.
However, in this system, there is a property in which even when the same signal voltage is applied in order to control the current, when the threshold value, and the on-resistance are varied, the driving current of the EL device is changed, and thus TFTs with less unevenness and having uniform characteristics are required.
As a transistor suitable for realizing such a driving circuit, there is a low temperature polysilicon TFT having a high mobility, using a user annealing process, and applicable to a large-type substrate. However, it is known that it has unevenness in the device characteristics, and when it is used as the organic EL device driving circuit, due to the unevenness of the TFT characteristics, even when the same signal voltage is applied, the unevenness in the brightness occurs in each pixel, and it has not been sufficient to display the gray scale with high precision.
Also, in JP-A-10-232649, as a driving method, the pixel is made to digitally and binary display the on/off state. As a result, since it is not necessary to use as the operating point, the neighbor of the threshold value at which the unevenness of the TFT characteristics reflects on the display significantly, there is a merit of reducing the unevenness of the brightness of the pixel. In order to obtain the gray scale display, one-frame time is divided into 8-subframes of different display times, and the average brightness is controlled by changing the light emission time.
In the digital driving system mentioned above, it is necessary to provide within the pixel a memory circuit capable of holding data of frame time or longer, and for stable memory operation, about seven transistors are necessary. However, in a pixel whose area is limited, when many transistors are included, the aperture ratio will be decreased, and when intended to obtain high resolution, the area for arranging the circuit will need 3 times as large as the analog pixel, and the high resolution becomes impossible.
An object of the present invention is to overcome the problems in the conventional technique mentioned above, and simplify the memory circuit built-in the pixel, and to provide an emissive display which has an increased aperture ratio, and high resolution.
Another object of the present invention is to provide an emissive display providing reduced power consumption of the circuit of the display.
To achieve the above-mentioned object, as to two sets of inverter circuits constituting a memory circuit arranged in each pixel, a circuit connecting an organic EL device and a transistor in series is used as one set of inverter circuit, thereby omitting a transistor in the memory circuit, simplifying the circuit, and improving the aperture ratio.
Furthermore, in the mutual connection of the two sets of inverters, by connecting so that display data is input to a line connected to a gate of the transistor connected in series with the EL device, it is possible to reduce a write load, to enable to write at high speed, and to obtain high resolution.
Furthermore, by forming a circuit configuration connected so that no through current flows by using p-channel transistors for all the transistors in the pixel, it is possible to reduce the power consumption at the memory holding period. Also, since it is possible to reduce the leakage current at the memory period, the power consumption of the circuit can be reduced.
The operation of the present invention will be explained. In the memory circuit arranged within the pixel, since the organic EL device operates as a diode, the driving transistor is connected in series, and it operates as a load device in the inverter. By this arrangement, an inverter circuit is formed, and by combining with another set of inverter circuit formed by only the CMOS transistors, it functions as a memory circuit.
In the writing of data to the pixel memory, by inputting the data so that the data is written to the gate of the driving transistor, since the gate capacitance is small, a driving load is reduced and high speed writing becomes possible.
Other objects, features and advantages of the present invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.